Sedimentation velocity experiments showed that only monomers coexist with amyloid fibrils of human islet amyloid-polypeptide (hIAPP or amylin. No oligomers containing less than 100 monomers could be detected, suggesting that the putative toxic oligomers are much larger than those found for the Alzheimers peptide, A&#946;(1-42). Extensive studies comparing the end-to-end contact kinetics of rat and human amylin using our tryptophan triplet quenching technique, together with molecular dynamics simulations, suggest that complex formation involving the disulfide loop at least partly explains why rat amylin does not aggregate to form amyloid fibrils.